Gas distribution apparatus for semiconductor processing

ABSTRACT

A gas distribution system for uniformly or non-uniformly distributing gas across the surface of a semiconductor substrate. The gas distribution system includes a support plate and a showerhead which are secured together to define a gas distribution chamber therebetween. A baffle assembly including one or more baffle plates is located within the gas distribution chamber. The baffle arrangement includes a first gas supply supplying process gas to a central portion of the baffle chamber and a second gas supply supplying a second process gas to a peripheral region of the baffle chamber. Because the pressure of the gas is greater at locations closer to the outlets of the first and second gas supplies, the gas pressure at the backside of the showerhead can be made more uniform than in the case with a single gas supply. In one arrangement, the first and second gas supplies open into a plenum between a top baffle plate and a temperature controlled support member wherein the plenum is divided into the central and peripheral regions by an O-ring. In a second arrangement, the first gas supply opens into the central region above an upper baffle plate and the second gas supply opens into the periphery of a plenum between the upper baffle plate and a lower baffle plate.

FIELD OF THE INVENTION

The present invention relates to reaction chambers used for processingsemiconductor substrates, such as integrated circuit wafers, andspecifically to improvements in the gas distribution system used inthese reaction chambers.

BACKGROUND OF THE INVENTION

Semiconductor processing includes deposition processes such as chemicalvapor deposition (CVD) of metal, dielectric and semiconductingmaterials, etching of such layers, ashing of photoresist masking layers,etc. In the case of etching, plasma etching is conventionally used toetch metal, dielectric and semiconducting materials. A parallel plateplasma reactor typically includes a gas chamber including one or morebaffles, a showerhead electrode through which etching gas passes, apedestal supporting the silicon wafer on a bottom electrode, an RF powersource, and a gas injection source for supplying gas to the gas chamber.Gas is ionized by the electrode to form plasma and the plasma etches thewafer supported below the showerhead electrode.

Showerhead electrodes for plasma processing of semiconductor substratesare disclosed in commonly assigned U.S. Pat. Nos. 5,074,456; 5,472,565;5,534,751; and 5,569,356. Other showerhead electrode gas distributionsystems are disclosed in U.S. Pat. Nos. 4,209,357; 4,263,088; 4,270,999;4,297,162; 4,534,816; 4,579,618; 4,590,042; 4,593,540; 4,612,077;4,780,169; 4,854,263; 5,006,220; 5,134,965; 5,494,713; 5,529,657;5,593,540; 5,595,627; 5,614,055; 5,716,485; 5,746,875 and 5,888,907.

A common requirement in integrated circuit fabrication is the etching ofopenings such as contacts and vias in dielectric materials. Thedielectric materials include doped silicon oxide such as fluorinatedsilicon oxide (FSG), undoped silicon oxide such as silicon dioxide,silicate glasses such as boron phosphate silicate glass (BPSG) andphosphate silicate glass (PSG), doped or undoped thermally grown siliconoxide, doped or undoped TEOS deposited silicon oxide, etc. Thedielectric dopants include boron, phosphorus and/or arsenic. Thedielectric can overlie a conductive or semiconductive layer such aspolycrystalline silicon, metals such as aluminum, copper, titanium,tungsten, molybdenum or alloys thereof, nitrides such as titaniumnitride, metal suicides such as titanium silicide, cobalt silicide,tungsten silicide, molybdenum silicide, etc. A plasma etching technique,wherein a parallel plate plasma reactor is used for etching openings insilicon oxide, is disclosed in U.S. Pat. No. 5,013,398.

U.S. Pat. No. 5,736,457 describes single and dual “damascene”metallization processes. In the “single damascene” approach, vias andconductors are formed in separate steps wherein a metallization patternfor either conductors or vias is etched into a dielectric layer, a metallayer is filled into the etched grooves or via holes in the dielectriclayer, and the excess metal is removed by chemical mechanicalplanarization (CMP) or by an etch back process. In the “dual damascene”approach, the metallization patterns for the vias and conductors areetched in a dielectric layer and the etched grooves and via openings arefilled with metal in a single metal filling and excess metal removalprocess.

It is desirable to evenly distribute the plasma over the surface of thewafer in order to obtain uniform etching rates over the entire surfaceof the wafer. Current gas distribution chamber designs include multiplebaffles which are optimized to uniformly distribute etching gas toachieve the desired etching effect at the wafer. However, the currentbaffle and showerhead electrode designs are best suited to empiricaloptimization for uniform gas distribution for a particular gap betweenthe wafer and the showerhead electrode and are difficult to adjust tovarying gaps between the wafer and the showerhead. In addition,conventional gas distribution designs include baffles having hundreds ofopenings or complex, difficult to manufacture geometries to ensure evendistribution of etching gas to the backside of the showerhead electrode.When etching large, twelve-inch (300 mm) wafers, controlling the processgas to create a uniform pressure distribution across the showerhead iseven more difficult. The number of openings and baffles must beincreased significantly to maintain uniform distribution of the etchinggas. As the number of openings in the baffles increase and the number ofbaffles increase, the complexity and cost to manufacture such a gasdistribution apparatus increase greatly.

SUMMARY OF THE INVENTION

The present invention provides a gas distribution system which is asimple to manufacture design requiring a small number of baffle plates,while still achieving desired gas distribution delivered through ashowerhead. Gas flow can be optimized for any size substrate and/or gapbetween the showerhead and the semiconductor substrate being processed.In addition, the present invention can improve heat transfer from ashowerhead electrode to a cooled support plate, thereby creating bettertemperature uniformity across the electrode surface. Furthermore, thepresent invention can provide generally continuous electrical contactamong the components of a showerhead electrode gas distribution system.

A gas distribution apparatus according to the present invention includesa support plate and a showerhead which are secured to define a gasdistribution chamber. The chamber includes a baffle assembly includingone or more baffle plates which can be used to achieve a desiredpressure distribution across the showerhead. Multiple gas suppliesprovide process gas into the gas distribution chamber where the processgas flows downward through the baffle assembly and through theshowerhead.

A first embodiment of the invention includes a baffle assembly having anupper baffle plate. A seal member, such as an O-ring is at anintermediate location between the upper baffle plate and the supportplate. The seal member divides the space therebetween into inner andouter regions. Gas from a first gas supply directs gas into the innerregion and gas from a second gas supply directs gas into the outerregion. The arrangement allows different gas chemistries and/or gaspressures to be provided to the inner and outer regions. As a result,better control of gas chemistry and/or gas pressure across the substratecan be achieved by preselecting process parameters or adjusting suchprocess parameters during processing of a substrate.

If desired, middle and/or lower baffle plates can be arranged to definethree plenums. The first plenum is located between the upper and middlebaffle plates. The second plenum is located between the middle and lowerbaffle plates, and the third plenum is located between the lower baffleplate and the showerhead. The plenums can be used to create a moreuniform process gas pressure distribution across the showerhead.

In a second embodiment of the present invention the support memberincludes a recess in its lower side which defines the gas distributionchamber. The support member has a first gas outlet supplying a firstprocess gas into a central area of the recess chamber and a second gasoutlet supplying a second process gas into a peripheral area of therecess. Secured within the baffle chamber are an upper baffle plate anda lower baffle plate. The upper baffle plate is arranged to receive gasexclusively from the first gas supply and the lower baffle plate isarranged to receive gas exclusively from the second gas supply. A firstset of gas passages in the upper baffle plate is in fluid connectionwith gas passages in the second baffle plate to create a set offlow-connected passages through which the first process gas passesdirectly from the upper baffle plate to the underside of the lowerbaffle plate. The second process gas flows through a second set of gaspassages in the lower baffle plate to its underside adjacent thebackside of the showerhead. In this arrangement, the first process gasdoes not mix substantially with the second process gas before flowing tothe underside of the lower baffle. The space between the lower baffleand the showerhead can have spaced apart annular channels which allowthe gases passing through the showerhead to be selectively controlled,e.g., to achieve uniform or nonuniform gas chemistry and/or pressureacross the showerhead. Gas from both the first gas supply and the secondgas supply flows through a third set of openings in the showerhead intoa region spanning the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be understood byreading the following detailed description in conjunction with thedrawings in which:

FIG. 1 is a sectional view of a gas distribution chamber according tothe present invention;

FIG. 2 is an exploded perspective sectional view of a first embodimentof the present invention;

FIG. 3 is a sectional view of the first embodiment of the presentinvention;

FIG. 4 is an exploded perspective view of a second embodiment of thepresent invention;

FIG. 5 is a sectional view of the second embodiment;

FIG. 6 is a perspective sectional view of a lower baffle plate of thesecond embodiment of the present invention;

FIGS. 7A-B show an etching process which can be carried out with the gasdistribution system of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the invention, the following detaileddescription refers to the accompanying drawings, wherein preferredexemplary embodiments of the present invention are illustrated anddescribed. In addition, the reference numbers used to identify likeelements in the drawings are the same throughout.

According to the present invention, process gas can be uniformlydistributed from one or more gas supplies to a substrate positionedunderneath a showerhead. The showerhead can be used in any type ofsemiconductor processing apparatus wherein it is desired to distributeprocess gas over a semiconductor substrate. Such apparatus includes CVDsystems, ashers, capacitive coupled plasma reactors, inductive coupledplasma reactors, ECR reactors, and the like.

A gas distribution system for a parallel plate plasma reactor is shownin FIG. 1 wherein a support plate 20 and a showerhead 22 are securedtogether to define a sealed gas distribution chamber 24. A baffleassembly 26, including one or more baffle plates, is located between thesupport plate 20 and the showerhead 22. According to the presentinvention, the geometry and arrangement of the baffle assembly 26 isconfigured to uniformly distribute gas to a backside 28 of theshowerhead 22. In semiconductor wafer processes such as chemical vapordeposition or dry-etch plasma processes, the controlled distribution ofprocess gas across the substrate is desirable in order to increase theconsistency and yield of these processes.

As seen in FIGS. 2 and 3, in a first embodiment of the present inventionthe baffle assembly 26 includes baffle plate 30A and optional baffleplates 30B and 30C. The baffle plates 30A-30C, are positioned within arecess 32 defined by a peripheral upwardly-projecting side 34 of theshowerhead 22. The upper baffle plate 30A is spaced from a bottomsurface 36 of the support plate 20 by an O-ring 38. The O-ring 38divides space between the upper baffle plate 30A and the support plate20 into two regions, each of which can be supplied process gas havingdifferent gas chemistries, pressures and/or flow rates. Gas from a firstgas supply 40 flows into a central region 42 between the upper baffleplate 30A and the support plate 20. Gas from a second gas supply 44flows into an annular channel 44 a and then into a peripheral region 46between the upper baffle plate 30A and the support plate 20. The middleand bottom plates 30B, 30C can be arranged below the upper baffle plate30A to define open plenums 48A, 48B therebetween and an open plenum 48Cbetween the bottom baffle plate 30C and the showerhead 22.

Each gas supply creates a pressure distribution across the surface ofthe upper baffle plate 30A wherein the gas pressure is highest adjacentthe gas supply outlet and decreases in a direction away from the outlet.Thus, the relative gas pressures between the peripheral 46 and central42 regions of the top surface of the upper baffle plate 30A can beadjusted using first and second mass flow controllers 50A, 50B which areconnected to the first and second gas supplies 40, 44. Each mass flowcontroller 50A, 50B can be supplied a desired gas mixture by adjustingflow rates of two or more gases supplied from gas supplies 50C, 50D,50E, 50F, etc.

Process gas is distributed across the central region 42 and peripheralregion 46 between the upper baffle plate 30A and the support plate 20,and passes through openings 52A in the upper baffle plate 30A into theopen plenum 48A between the upper and middle baffle plates 30A, 30B.Thereafter, gas flows downward through openings 52B in the middle baffleplate 30B into an open plenum 48B between the middle and bottom baffleplates 30B, 30C, then through openings 52C in the bottom baffle plate30C into an open plenum 48C between the bottom baffle plate 30C and theshowerhead 22, and ultimately through openings 54 in the showerhead 22before reaching a substrate. Each time the gas enters into an openplenum, nonuniform pressure distribution is damped as any nonuniformpressure equalizes somewhat from areas of high pressure to areas of lowpressure. Thus, by configuring the gas distribution system to define aplurality of plenums 48 between the baffle plates 30, a substantiallyuniform pressure distribution can be achieved at the backside 28 of theshowerhead 22.

A second embodiment of the gas distribution system is shown in FIGS. 46.The baffle assembly of the second embodiment includes two baffle plates56A, 56B. The upper baffle plate 56A includes portions in contact withthe support plate 20 and the lower baffle plate 56B includes portions incontact with the showerhead 22. Surface to surface contact between thesupport plate 20, baffle assembly 26 and the showerhead 22 bothfacilitates heat transfer between the showerhead 22, the baffle assembly26 and the support plate 20, and can provide an electrically conductivepath between the showerhead 22, baffle assembly 26 and the support plate20 in the case where the showerhead is used as a top electrode.

During processing, the temperature controlled support plate 20 acts as aheat sink, drawing heat from the showerhead 22 through the baffleassembly 26. For instance, coolant can be circulated through coolingchannels 58 in the support plate 20 to dissipate heat generated duringprocessing of a substrate.

In the second embodiment, a first gas supply 60 is configured to feedgas to a central recess 62 in the upper baffle plate 56A. A second gassupply 64 feeds gas to an annular manifold 66 which distributes gas to aperipheral region 68 above the lower baffle plate 56B. The manifold 66may be integral with the support plate 20 or can comprise a separatecomponent of the gas distribution system.

The upper baffle plate 56A includes radially extending channels 70 whichdistribute gas from the generally centrally located first gas supply 60to the periphery of the upper baffle plate 56A. The channels 70 aredefined between contact surfaces 72 which contact the bottom surface 36of the support plate 20. Heat and electric current flows from the upperbaffle plate 56A to the support plate 20 through the surfaces 72.Similarly, the top surface of the lower baffle plate 56B includesradially extending channels 74 which distribute gas from theperipherally located manifold 66 to an annular channel 76 in a centralpart of the lower baffle plate 56B. The radially extending channels 74are defined between contact surfaces 78 which thermally and electricallycontact the upper baffle plate 56A. Although the channels 70, 74 and 76are shown in the upper surfaces of the upper and lower baffles, theycould also be formed in lower surfaces of the support plate 20 and upperbaffle plate.

Openings 80 located in the radially extending channels 70 in the upperbaffle plate are flow-connected to a first set of openings 82 in thelower baffle plate 56B. That is, the openings 80 in the upper baffleplate 56A and the first set of openings 82 in the lower baffle plate 56Bdefine a generally continuous and uninterrupted fluid pathway from thefirst gas supply 60 through the upper and lower baffle plates 56A, 56B.Gas from the second gas supply 64 flows through a second set of openings84 in the channels 74 in the lower baffle plate 56B. The flow-connectedopenings 80, 82 and the second set of openings 84 are arranged toprevent significant mixing between gas from the first gas supply 60 andthe second gas supply 64. Such an arrangement will allow some gas tomigrate between the upper and lower baffle plates. To prevent suchmigration, the upper and lower baffle plates could be adhesively ormetallurgically bonded together in a manner which prevents the two gasesfrom mixing together.

Preferably, the flow-connected openings 80, 82 are created by aligningthe openings 80 in the upper baffle plate with the first set of openings82 in the lower baffle plate by any suitable technique such as matingalignment features such as locating pins. However, other techniques forconnecting openings 80 to openings 82 include interposition of apatterned gasket between the upper and lower baffles or the provision ofindividual tubes bonded between the openings in the upper and lowerbaffle plates.

The bottom surface of the lower baffle plate 56B includes annulardownwardly-projecting wall portions 86 which thermally and electricallycontact the top surface of the showerhead 22. Both the flow-connectedopenings 80, 82 and the second set of openings 84 open into radiallyspaced annular channels 88 defined by the downwardly-projecting wallportions 86. The channels 88 could be formed in the upper surface of theshowerhead or the space between the lower baffle plate and theshowerhead could be an open plenum with or without contact portionstherebetween for conducting heat away from the showerhead and/orsupplying electrical power to the showerhead.

During semiconductor processing, gas from the first gas supply 60 flowsthrough flow-connected openings 80, 82 in the upper baffle plate 56A andthe lower baffle plate 56B, and gas from the second gas supply 64 flowsthrough the second set of openings 84 in the lower baffle plate 56B. Gasfrom both the first and second gas supplies 60, 64 mixes in the channels88 in the underside of the lower baffle plate above the top surface ofthe showerhead 22 and flows through a third set of openings 90 in theshowerhead 22 toward the substrate.

Across the upper baffle plate 56A, gas pressure is highest near thecentrally located first gas supply 60 and lowest near the periphery ofthe upper baffle plate 56A. Process gas flows downward through theflow-connected openings 82, 84 in the upper and lower baffle plates 56A,56B to the open channels 88 in the underside of the lower baffle plate56B. In operation, if the first and second gas supplies deliver gas atthe same pressure, gas from the first gas supply 60 sets up a pressuredistribution wherein pressure is high proximate to the center of thelower baffle plate 56B and low at the periphery of the lower baffleplate 56B whereas the gas from the second gas supply 64 sets up apressure distribution wherein pressure is high at the periphery and lowat the center of the lower baffle. Consequently, with the bafflearrangement of the invention, the pressure seen at the backside of theshowerhead can be made more uniform across the backside of theshowerhead.

In an alternative processing scheme, the gas distribution system canprovide a controlled, nonuniform gas pressure distribution across thebackside 28 of the showerhead 22. For example, if high gas pressurearound the periphery of the backside 28 of the showerhead 22 isdesirable, the flow through the second gas supply 64 can be selectivelyincreased relative to the flow through the first gas supply 60.Conversely, if relatively high gas pressure is desirable near the centerof the backside 28 of the showerhead 22, the flow through the first gassupply 60 can be increased relative to the flow through the second gassupply 64. Thus, in the case of single wafer processing, the gasdistribution system can supply different gas chemistries to one or moreannular zones above the wafer. Because the gas chemistry, flow rate andpressure can be made uniform circumferentially around each such annularzone but varied radially from zone to zone it is possible to effectuniform processing of a wafer during processes wherein the processingconditions at the wafer surface vary across the wafer.

FIGS. 7A-B show schematics of how a dual-damascene structure can beetched in a single step in accordance with the invention. FIG. 7A showsa pre-etch condition wherein an opening 500 corresponding to a trench isprovided in a photoresist masking layer 520 which overlies a stack of afirst dielectric layer 540 such as silicon oxide, a first stop layer 560such as silicon nitride, a second dielectric layer 580 such as siliconoxide, a second stop layer 600 such as silicon nitride, and a substrate620 such as a silicon wafer. In order to obtain etching of vias throughthe first stop layer 560 in a single etching step, first stop layer 560includes an opening 640. FIG. 7B shows the structure after etchingwherein the opening 500 extends through the dielectric layer 540 to thefirst stop layer 560 and the opening 640 extends through the seconddielectric 580 to the second stop layer 600. Such an arrangement can bereferred to as a “self-aligned dual-damascene” structure.

During the etch process, process gas conditions supplied by the firstand second gas supplies in the first and second embodiments can bechanged relative to each other, e.g., during etching of the trench 500 amixture of Ar, oxygen and fluorocarbons (e.g., CHF₃ and C₄F₈) can besupplied and during etching of the vias 640 the flow of the oxygen tothe central region of the wafer can be decreased. In the case of etchinglow-k dielectric layers, the process gas can include a hydrocarbon suchas C₂H₄ and the hydrocarbon to oxygen gas flow rate ratio can be variedradially to achieve uniform etching. Thus, according to the inventionthe flow of gases to the center and edge of the wafer can be adjusted tocompensate for edge fast etching and center fast etching conditions inthe plasma chamber. For example, in a conventional plasma etcher, edgefast etch conditions can occur until the photoresist is eroded afterwhich center fast etch conditions can occur. With the gas distributionapparatus according to the invention, more oxygen can be supplied in thecenter when the wafer has a photoresist layer whereas when thephotoresist layer is eroded away, the flow of oxygen to the center canbe reduced. As a result, more uniform etching can be achieved bycompensating for the edge-fast and center-fast etch conditions.

The process of the invention is applicable to various plasma processesincluding plasma etching of various dielectric layers such as dopedsilicon oxide such as fluorinated silicon oxide (FSG), undoped siliconoxide such as silicon dioxide, spin-on-glass (SOG), silicate glassessuch as boron phosphate silicate glass (BPSG) and phosphate silicateglass (PSG), doped or undoped thermally grown silicon oxide, doped orundoped TEOS deposited silicon oxide, etc. The dielectric dopantsinclude boron, phosphorus and/or arsenic. The dielectric can overlie aconductive or semiconductive layer such as polycrystalline silicon,metals such as aluminum, copper, titanium, tungsten, molybdenum oralloys thereof, nitrides such as titanium nitride, metal silicides suchas titanium silicide, cobalt silicide, tungsten silicide, molydenumsilicide, etc.

The plasma can be a high density plasma produced in various types ofplasma reactors. Such plasma reactors typically have high energy sourceswhich use RF energy, microwave energy, magnetic fields, etc. to producethe high density plasma. For instance, the high density plasma could beproduced in a transformer coupled plasma (TCPT) which is also calledinductively coupled plasma reactor, an electron-cyclotron resonance(ECR) plasma reactor, a helicon plasma reactor, or the like. An exampleof a high flow plasma reactor which can provide a high density plasma isdisclosed in commonly owned U.S. Pat. No. 5,820,723, the disclosure ofwhich is hereby incorporated by reference.

The present invention has been described with reference to preferredembodiments. However, it will be readily apparent to those skilled inthe art that it is possible to embody the invention in specific formsother than as described above without departing from the spirit of theinvention. The preferred embodiment is illustrative and should not beconsidered restrictive in any way. The scope of the invention is givenby the appended claims, rather than the preceding description, and allvariations and equivalents which fall within the range of the claims areintended to be embraced therein.

What is claimed is:
 1. A gas distribution system useful for a reactionchamber used in semiconductor substrate processing, comprising: asupport member having a recess in a lower surface thereof, the supportmember having a first gas supply opening into a central area of therecess and a second gas supply opening into a peripheral area of therecess; a baffle arrangement located in the recess, the bafflearrangement including first and second openings arranged such that gasfrom the first gas supply passes through the first openings withoutpassing through the second openings in the baffle arrangement and gasfrom the second gas supply passes through the second openings withoutpassing through the first openings in the baffle arrangement; and ashowerhead supported by the support member such that the gas passingthrough the first and second openings mixes together and passes througha third set of openings in the showerhead.
 2. The gas distributionsystem of claim 1, wherein the showerhead is a top electrode and thesupport member is a temperature-controlled member of a plasma reactionchamber.
 3. The gas distribution system of claim 1, wherein the supportmember comprises a support ring attached to a temperature-controlledmember.
 4. The gas distribution system of claim 1, wherein theshowerhead comprises a showerhead electrode.
 5. The gas distributionsystem of claim 1, further comprising a first mass flow controllerconnected to the first gas supply, a second mass flow controllerconnected to the second gas supply, and a controller connected to thefirst and second mass flow controllers so as to adjust gas chemistryand/or flow rates of the process gas supplied by the first and secondgas supplies.
 6. The gas distribution system of claim 1, wherein thebaffle arrangement comprises upper and lower baffle plates wherein thefirst openings are located in the upper baffle plate and in the lowerbaffle plate so as to define a continuous and uninterrupted fluidpathway through the upper and lower baffle plates.
 7. The gasdistribution system of claim 1, wherein the baffle arrangement includesupper and lower baffle plates, the gas passing through the first andsecond openings mixes in gas flow channels located between the lowerbaffle plate and the showerhead.
 8. The gas distribution system of claim7, wherein the channels are formed in a lower surface of the lowerbaffle plate and/or in an upper surface of the showerhead, the lowersurface of the lower baffle plate being in contact with the uppersurface of the showerhead.
 9. The gas distribution system of claim 7,wherein the showerhead is an electrode, the upper and lower baffleplates are of an electrically conductive material and the channels areformed in a lower surface of the lower baffle plate and/or in an uppersurface of the showerhead, the lower surface of the lower baffle platebeing in electrical and thermal contact with the upper surface of theshowerhead.
 10. The gas distribution system of claim 1, wherein thebaffle arrangement includes upper and lower baffle plates and the secondgas supply supplies gas to one or more gas flow channels located betweenthe upper and lower baffle plates, the gas from the second gas supplyflowing through the channels in a direction from an outer region of thebaffle plates towards an inner region of the baffle plates.
 11. The gasdistribution system of claim 10, wherein the channels are formed, in alower surface of the upper baffle plate and/or in an upper surface ofthe lower baffle plate.
 12. The gas distribution system of claim 11,wherein the upper surface of the lower baffle plate is in thermalcontact with the lower surface of the upper baffle plate.
 13. The gasdistribution system of claim 11, wherein the showerhead is an electrode,the upper and lower baffle plates are of an electrically conductivematerial and the upper surface of the lower baffle plate is inelectrical contact with the lower surface of the upper baffle plate. 14.A gas distribution system useful for a reaction chamber used insemiconductor substrate processing, comprising: a support member havinga recess in a lower surface thereof, the support member having a firstgas supply opening into a central area of the recess and a second gassupply opening into a peripheral area of the recess; a bafflearrangement located in the recess such that gas from the first gassupply passes through first openings in the baffle arrangement and gasfrom the second gas supply passes through second openings in the bafflearrangement; and a showerhead supported by the support member such thatthe gas passing through the first and second openings mixes together andpasses through a third set of openings in the showerhead; wherein thebaffle arrangement includes a baffle plate and a seal member, the sealmember separating a space between the baffle plate and the supportmember into central and peripheral regions, the first gas supply openinginto the central region and the second gas supply opening into theperipheral region.
 15. The gas distribution system of claim 14, whereinthe seal member is an O-ring.